It is not Sunnah to wear black when anyone dies, hence the scholars regarded it as bid’ah.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was asked: what is the ruling on women attending funerals? And on wearing black clothes?
It is haraam for women to attend funerals because they have little patience and because it exposes them to fitnah and mixing with men. With regard to wearing black clothes at times of calamity, this is an innovation.
Majmoo’ Fataawa Ibn ‘Uthaymeen, 17/329.
He was also asked (17/410): what is the ruling on singling out certain kinds of clothes for offering condolences such as women wearing black?
Singling out a certain type of clothes for offering condolences is an innovation as far as we know, and it may stem from a person’s being displeased with the decree of Allaah. Although some people think there is nothing wrong with it, the salaf did not do this and as it is to some extent an expression of displeasure with the decree of Allaah, undoubtedly it is better not to do it, because if a person does that he may be closer to sin than not.
He also said (17/414):
Wearing black in mourning for the dead is a kind of bid’ah and displaying grief, and it is akin to rending one’s garment and slapping one’s cheeks, and the Prophet (peace and blessings of Allaah be upon him) disavowed the one who does that when he said: “He is not one of us who rends his garment and slaps his cheeks and calls out with the call of jaahiliyyah.”.